The most important porous mineralic bodies are silica gel, activated alumina and molecular-sieve zeolites, used preferentially as adsorbents. In comparison with carbon-containing adsorbents, they are characterized by a high oxidation resistance and highly hydrophilic properties. Silica gel is a solid precipitated from colloidal systems, consisting essentially of amorphous silicic acid and water. Upon its dehydration, secondary structures of coherent polysilicic acids are permeated by a system of pores of different sizes and cross-sections whose width depends on the pH of the starting solutions. The dehydrated silica gels constitute materials of significant adsorptivity for water and other polar substances.
Activated alumina is a porous form of aluminum oxide obtained from precipitated alumina hydrates by calcination in a temperature range between 200.degree. and 800.degree. C. Different types of alumina can be distinguished according to their properties regarding grain structure, adsorptivity, purity and mechanical strength. An increase of their effective surface is achieved by the addition of certain interacting cations such as those of calcium.
In the case of molecular-sieve zeolites one can distinguish between natural and synthetic zeolites which can be regarded as differently hydrated aluminum silicates. The most significant property of the aluminum silicates is the fact that their stepwise dehydration occurs without a change in their crystal structure. The resulting regular lattice configuration with pores open to the internal cavities of the crystal lattice constitutes the pore volume which is essential for adsorption. Three basic methods are available for the manufacture of industrial zeolites, namely a purification of natural zeolites, a recrystallization of natural minerals and a direct synthesis from the components of zeolites, namely the processing of aluminates, silicates, alkalies and alkaline earths in an aqueous medium.
Direct synthesis most often relies upon recrystallization which is also used for the manufacture of porous aluminum oxides. This technique consists in first obtaining amorphous precipitates from the aforementioned components which can be partly or completely converted into crystalized products by an after-treatment such as a hydrothermal process or controlled sintering. The various types of zeolites are obtainable by modifying the concentration of the starting materials and by a choice of after-treatment. Further alternatives in the production of molecular sieves include the replacement of the pure starting materials by cheaper substances or waste products, such as clay, kaolin, and fly ash from filter stages of power plants. The utilization of such low-cost materials has been described, for example, in G.D.R. (East German) patents WP No. 156,592, relating to adsorbents for utilization in thermal panes and glazing units, and WP No. 156,254, pertaining to a process for the production of an aluminum-silicate adsorbent.
Other porous mineralic bodies, of lesser significance, are porous glasses and porous thorium oxide. Porous glasses are produced from a specially constituted boron/silicate glass by annealing and subsequent treatment with 3-normal hydrochloric acid. Porous thorium oxide is manufactured by the well-known sol/gel process. In the production of special cores of thorium oxide for high-temperature reactors, soot or graphite is dispersed in the gel and is removed with the aid of atmospheric oxygen after a sintering process.
A drawback of the conventional porous mineralic bodies is their low mechanical and thermal stability. Their use as drying agents, catalyst support or adsorbents in various technical processes generally involves an exposure to more or less elevated temperatures. Particular problems exist when a thermal regeneration or reactivation of the material is essential for reasons of economy an efficiency.
In the case of activated alumina and silica gel, a risk of impairment exists already at temperatures somewhat above 300.degree. C. If, for example, activated alumnia is subjected to temperatures above 400.degree. C., its active surface shrinks. A temperature of 600.degree. C. reduces the specific surface of alumina from 300 to 200 m.sup.2 /g. Molecular-sieve zeolites, which have the highest thermal stability among the substances referred to above, are stable up to a temperature of 600.degree. C. or, with a few types, 800.degree. C. A disadvantage of these molecular sieves, however, is that their uniform pore system makes them unstable for wastewater purification, as does their high affinity for water due to their hydrophilic properties.
An adsorbent described in the aforementioned G.D.R. patent WP No. 156,254 is a modified molecular sieve which is produced by a hydrothermal treatment of power-plant filter ash and thus has the same drawbacks as normal molecular sieves. The other G.D.R. patent referred to, WP No. 156,592, describes the utilization of such a material for the recovery of water vapor from inflation gases.
Porous glasses and thorium oxide cannot be widely used for adsorption and purification purposes, on account of their high material and production costs. Also disadvantageous is the extreme thermal sensitivity of thorium oxide manufactured by the sol/gel process.